Assay-guided fractionation study of α-amylase inhibitors from Garcinia mangostana pericarp

Abstract

α-Amylase inhibitor (α-AI) activity of Garcinia mangostana, commonly known as mangosteen, pericarp extracts was studied by assay guided fractionations from lipophilic to hydrophilic using combined solvent extraction and Amberlite XAD2 adsorption chromatography. Neither the lipophilic, xanthone containing fraction, nor the highly polar fraction, which has no affinity on Amberlite XAD2, showed any α-AI. The fraction that shows very high inhibitory activity contains primarily polyphenols and can be adsorbed on Amberlite XAD2. The IC50 of 5.4 μg/mL of this fraction is comparable to that of acarbose, a prescribed α-AI used in the control of type II diabetes, at 5.2 μg/mL. Total phenolic content (TPC) of each fraction was measured and the TPC has no correlation with the α-AI activity. The lipophilic fraction contains mainly xanthones as revealed by HPLC-MS analysis. Colorimetric analysis coupled with UV-vis and IR spectroscopic analysis demonstrated that the fractions with high α-AI activity are primarily oligomeric proanthocyanidins (OPCs) with little gallate moiety. There is also evidence to show that the α-AI by these OPCs is not purely by nonspecific protein complexation. Both tannic acid and G. mangostana OPCs precipitate BSA equally well but G. mangostana OPCs are 56 times more effective in inhibiting α-amylase. © 2007 American Chemical Society.